Earth moving equipment, such as a motor grader for example, often must operate in environments with poor footing conditions. Some earth moving machines are equipped with an all wheel drive system so that the front wheels of the machine, normally used for steering, are driven to help propel the machine in these poor footing conditions. Normally, only the rear wheels are driven, but when poor footing conditions are encountered causing excessive wheel slip, the front wheels of an all wheel drive machine can be driven to increase traction of the machine thereby reducing slip and maintaining directional control. When all wheels are in poor footing conditions, such as working on side slopes, or maximum traction is desired, the machine can be operated in an all wheel drive mode. Operating a motor grader in an all wheel drive mode for maximum traction may be desirable when the motor grader is performing a cutting operation, such as cutting a new road or grading on a side slope. The all wheel drive mode is also useful when a motor grader is used for removing snow from a roadway.
On a motor grader, a hydraulic pump is used to supply pressurized fluid to pump motors which drive the front wheels in the forward and reverse directions. A torque control lever is used in the all wheel drive mode to control the pressure and therefore the rim pull delivered to the front wheels.
When operating in the all wheel drive mode, the traction conditions can result in the front wheels actually hopping off the ground. These traction conditions can occur when operating in snow, sand, or firm underfoot conditions with a thin layer of loose material on top. This hopping condition causes loss of traction and in excess of twice the front axle weight upon impact. Hopping can occur when applying large amounts of torque to the from wheels and the front wheels encounter the poor traction conditions causing the front wheels to alternately spin and stick. It can be appreciated that it would be highly desirable to operate in an all wheel drive mode without fear of excessive hopping. In some instances, an operator may intentionally cause pressure fluctuations which alternately cause the wheels to stick and spin somewhat. Such operation is useful under certain conditions. It is desirable that the operator be able to cause or continue the pressure fluctuations to accomplish a given task.